The mammalian mitochondrial genome is a small, circular, double-stranded molecule containing 37 genes. Of these, 24 (22 tRNAs and two rRNAs) are needed for mitochondrial DNA translation, and 13 encode subunits of the respiratory chain complexes. Many mitochondrial processes are strictly dependent on the interaction between specific mitochondrial DNA sequences and the nuclear-encoded mitochondrial polypeptides with which they have co-evolved. Therefore, the majority of mitochondrial polypeptides are nuclear encoded, translated in the cytoplasm, and then transported across one or both of the mitochondrial membranes. It is thought that mitochondria contain more than 1,000 different proteins.